1883.] of electric currents. 51 



upon this number may be used with good effect when the strength 

 of the current ranges from -gL ampere to perhaps 4 amperes. It 

 requires however a pretty good balance, and some experience in 

 chemical manipulation. 



Another method which gives good results and requires only 

 apparatus familiar to the electrician, depends upon the use of a 

 standard galvanic cell. The current from this cell is passed 

 through a high resistance, such as 10,000 ohms, and a known 

 fraction of the electro-motive force is taken by touching this cir- 

 cuit at definite points. The current to be measured is caused 

 to flow along a strip of sheet German silver, from which two 

 tongues project. The difference of potential at these tongues is 

 the product of the resistance included between them and of the 

 current to be measured, and it is balanced by a fraction of the 



known electro-motive force of the standard cell (Fig. 1). With a 

 sensitive galvanometer the balance may be adjusted to about j^qq. 

 The German silver strip must be large enough to avoid heating. 

 The resistance between the tongues may be ^fa ohm, and may be 

 determined by a method similar to that of Matthiessen and Hockin 

 (Maxwell's Electricity, § 352). The proportions above mentioned 

 are suitable for the measurement of such currents as 10 amperes. 



Another method, available with the strong currents which are 

 now common, depends upon Faraday's discovery of the rotation of 

 the plane of polarization by magnetic force. Gordon found 15° 1 

 as the rotation due to the reversal of a current of 4 amperes circu- 

 lating about 1000 times round a column of bisulphide of carbon. 

 With heavy glass, which is more convenient in ordinary use, the 

 rotation is somewhat greater. With a coil of 100 windings we 

 should obtain 15° degrees with a current of 40 amperes ; and this 

 rotation may easily be tripled by causing the light to traverse the 

 column three times, or what is desirable with so strong a current, 

 the thickness of the wire may be increased and the number of 

 windings reduced. With the best optical arrangements the rotation 



1 [Jan. 1884. In a note recently communicated to the Eoyal Society (Proceedings, 

 Nov. 15, 1883) Mr Gordon points out that owing to an error in reduction, the number 

 given by him for the value of Verdet's constant is twice as great as it should be. The 

 rotations above mentioned must therefore be halved, a correction which diminishes 

 materially the prospect of constructing a useful instrument upon this principle.] 



4—2 



